1. Field of the Invention
The present invention generally relates to the conversion of chemical energy to electrical energy, and more particularly, to the treatment of a conductive substrate intended for subsequent use as a cathodic current collector. According to the present invention, heating in an oxidizing atmosphere or through electrolytic oxidation substantially thickens the naturally occurring passivating oxide surface layer on a metallic current collector. The thusly-conditioned conductive substrate improves the efficiency of a subsequently constructed cell, especially an alkali metal electrochemical cell discharged at an elevated temperature.
2. Prior Art
Titanium metal cathode current collectors generally exhibit the excellent corrosion resistance and other characteristics needed to function in the electrochemically challenging environments existing within lithium/carbon monofluoride (Li/CFx) cells. However, under certain storage and/or elevated temperature treatment or discharge conditions, use of titanium current collectors results in poor or degraded cell performance. For example, current collector and cell performance can be degraded by long term storage at ambient temperature, by short-term exposure to medical autoclave temperatures, and by high temperature discharge in commercial environments, such as down-hole in oil and gas wells. Degraded cell performance characteristics include voltage fluctuations during discharge, reduced running voltages, increased internal impedances, and reduced delivered capacities.
Prior art measures to avoid the adverse effects of titanium current collectors on cell performance resulting from storage or elevated temperature exposure have drawbacks. One such measure consists of coating an electrically conductive carbon paint suspension onto titanium cathode screens. However, variations in coating thickness and in adherence quality are but two of the factors making this a cumbersome process.
U.S. Pat. No. 5,670,278 to Disselbeck et al. discloses titanium supports and electrodes for primary and secondary electric cells. At column 7, lines 37 to 42, this patent teaches that if “the titanium-coated active-material support of the present invention is to be used for manufacturing negative electrodes, it is advantageous to effect the coating with a titanium which has been passivated with small amounts of oxidizing metal ions. Examples of such metal ions are Cu2+, Fe3+ and Cr4+.”
However, the presence of metallic ions other than those intended to promote electrochemical discharge can cause parasitic side reactions. In the present electrochemical system, lithium ions intercalate into the cathode active material where they are converted into atomic or molecular forms. The presence of Cu2+, Fe3+ and Cr4+ ions may interfere with these intended reactions as well as promote self-discharge and, consequently, depleted discharge capacity.
U.S. Pat. No. 5,114,810 to Frysz et al., which is assigned to the assignee of the present invention and incorporated herein by reference, discloses subjecting a titanium current collector screen to an atmosphere of 90% relative humidity for 30 hours then baking the screen in a 110° C. vacuum oven for 20 hours. However, the thusly-treated screen exhibited pitting corrosion. The likely reason for this is that the titanium oxide layer was too thick, and the differential stresses between it and the titanium base were too great. Having too large a disparity between the stress forces likely resulted in delamination of the titanium oxide layer from the base layer, which ultimately lead to corrosion of the titanium base. According to the present invention, the differential stresses between the passivating titanium oxide layer and the titanium base need to be as low as possible to prevent cracking and flaking leading to delamination and corrosion.
Another prior art measure is to abandon titanium and employ screens made from other highly corrosion resistant alloys such as high chromium ferritic stainless steels or cobalt-nickel alloys. Many of these alloys tend to be inherently expensive and difficult to form into screens by cost-efficient processes. Furthermore, the relatively lightweight of titanium is preferable and advantageous in many battery applications.
It is important to the understanding of the present invention to note that in the prior art involving the use of uncoated titanium current collectors, the titanium surface includes a very thin, invisible oxide layer that forms naturally on pure and alloyed titanium. This layer forms spontaneously in the presence of an oxygen source during mechanical and chemical forming processes including sheet/foil rolling, wire drawing, and chemical photo etch fabrication of screens. It is known to those familiar with the art that the oxide layer on titanium rapidly reaches a self-limited thickness under ambient or modestly elevated temperature conditions. It is, therefore, a premise of the present invention that conditioning the current collector in an oxidizing atmosphere increases the thickness of the titanium oxide layer.